This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Topoisomerases are essential enzymes that impact nearly every aspect of DNA metabolism through the maintenance of DNA supercoiling, untangling DNA knots, and resolving DNA catenanes. These enzymes are highly conserved in all organisms and are major pharmaceutical targets. Our current challenges focus on how human topoisomerase II alpha, a type-2 topoisomerase, untangles rather than tangles its DNA substrates. Although there are extensive biochemical data available for the enzyme, there exists no atomic structure information on any complete type-2 topoisomerase. Atomic structures are available of individual domains of eukaryotic type-2 enzymes, such as the ATPase domain for both the yeast and human enzymes and the cleavage-religation region of the yeast enzyme;however, there is no atomic structure information available for 22% of the C-terminus of the human protein. Electron cryomicroscopy and single particle reconstruction will be used to determine the three-dimensional structure of the complete enzyme alone or complexed with its DNA substrate in the presence and absence of anticancer drugs. In addition, reconstructions of isolated reaction intermediates will be used to determine the conformational changes of the enzyme. Human topoisomerase II alpha is a homodimer of 340 kDa and our initial multiple refinement convergence reconstructions of the enzyme alone show the dimensions of the particles to be roughly 65 A x 70 A x 75 A. With the structures of the enzyme alone, complexed with DNA, with anticancer drugs, and each trapped kinetic intermediate, we will be able to interpret how the machine moves and progresses through its catalytic cycle.